MISRA.STDLIB.MUTEX.DBL_LOCK.2023
Mutex variable is double locked
MISRA C 2012 AMD4 Rule 22.18 (Required): Non-recursive mutexes shall not be recursively locked.
Category: Required
Analysis: Undecidable, System
Applies to: C11
Amplification
A non-recursive mutex shall only be locked by a thread if it has not already been locked by that before.
Rationale
It is undefined behaviour if a non-recursive mutex is recursively locked by the calling thread. If the thread also attempts to unlock the mutex twice, the second call to mtx_unlock() will also result in undefined behaviour, since the mutex then will already be unlocked.
Example
Copy
mtx_t Ra;
mtx_t Rb;
int32_t t1( void *ignore ) /* Thread 1 */
{
mtx_lock ( &Rb ); /* Compliant */
mtx_lock ( &Rb ); /* Compliant - Rb is recursive */
mtx_unlock( &Rb ); /* Rb still locked */
mtx_unlock( &Rb ); /* Rb gets unlocked */
mtx_lock ( &Ra ); /* Compliant */
mtx_lock ( &Ra ); /* Non-compliant - undefined behaviour, deadlock possible */
mtx_unlock( &Ra ); /* If reachable (i.e. no deadlock), Ra gets unlocked */
mtx_unlock( &Ra ); /* Undefined behaviour if reachable */
return 0;
}
thrd_t id1;
thrd_t id2;
int32_t main(void)
{
mtx_init ( &Ra, mtx_plain );
mtx_init ( &Rb, mtx_recursive );
thrd_create( &id1, t1, NULL );
...
}See also
Dir 4.13, Rule 22.1, Rule 22.17
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